Epoxy-amine adhesives of superior toughness

ABSTRACT

THERMOSETTING EPOXY-AMINE ADHESIVE COMPOSTIONS WHICH HAVE A UNIFORMLY DISTRIBUTED CONTENT OF A WATERINSOLUBLE RUBBER POLYMER AT LEAST 10 MOL PERCENT COMPOSED OF EPICHLOROHYDRIN LINKAGES AND HAVING A MOLECULAR WEIGHT OF AT LEAST 100,000 POSSESS IMPROVED TOUGHNESS. THESE COMPOSTIONS FIND USE IN THE MANUFACTURE OF AIRCRAFT WING AND FUSELAGE SECTIONS.

United States Patent US. Cl. 260-830 R 9 Claims ABSTRACT OF THEDISCLOSURE Thermosetting epoxy-amine adhesive compositions which have auniformly distributed content of a waterinsoluble rubbery polymer atleast 10 mol percent composed of epichlorohydrin linkages and having amolecular weight of at least 100,000 possess improved toughness. Thesecompositions find use in the manufacture of aircraft wing and fuselagesections.

This is a continuation-in-part of our copending application Ser. No.169,500, filed on Aug. 5, 1971, now abandoned.

The present invention relates to novel thermosetting adhesivecompositions of the epoxy-amine type. The invention includes suchcompositions in bulk and in film and sheet form (with and withouttextile filler), and laminates bonded together by the composition inthermoset state.

Epoxy-amine adhesive compositions constitute an important and widelyused group of industrial high-strength adhesives. They consist ingeneral of a homogeneous mixture of one or more polyfunctional epoxycompounds and one or more polyfunctional amines as curing agenttherefor, or they may be a low-stage inter-reaction product of thesematerials. The composition form an adhesive of great tenacity whenhot-pressed between two surfaces so that they thermoset. Compositions ofthis type are disclosed in Encyclopedia of Chemical Technology by Kirk-Othmer (2nd Ed., vol. 8, pp. 204312, New York, 1965), and in US. Pats.Nos. 2,615,007; 2,915,490; 2,951,824; 2,986,546; 3,371,008; and3,386,955. A typical example of such a composition is a mixture of thediglycidyl ether of bisphenol A and ethylenediamine.

For convenience these materials are hereinafter termed thermosettingepoxy-amine adhesive compositions.

It is a disadvantage of these compositions that they are somewhatbrittle when thermoset. As a result, joints cemented with thesecompositions tend to fail when the joined articles are sharply struck orwhen the joints are subjected to prolonged vibration or peeling forces.Since a principal use for these compositions is in the cementing ofsheet aluminum for use as wing, fuselage and other sections of civilianand military aircraft, a demand has arisen for such compositions whichprovide a tougher and stronger bond.

The discovery has now been made that compositions of the class describedprovide tougher and stronger bonds when they have a uniformlydistributed content of a water-insoluble rubbery epichlorohydrinpolymer. In the composition the epichlorohydrin thus acts as latenttoughening agent and develops its toughening properties when theadhesive components of the composition are thermoset. In preferredinstances, amounts of the epichlorohydrin polymer in the minor range of10%25% produce increases in shear of better than 90% at roomtemperature, 60% at 180 F. and 80% at 67 F, and despite these increasesthey produce laminations which possess very good peel strength at thesetemperatures.

We have found that in preferred instances the invention provides thefollowing additional advantages:

1. The compositions are easily prepared in simple equipment, and nospecial techniques or controls need be utilized.

2. The compositions possess unusually long stability at roomtemperature.

3. They possess good cold tack for most industrial metal surfaces. Hencethe adhesive in sheet or film form is easily and conveniently applied tothe undersides of structural surfaces.

4. They possess a strong affinity for synthetic polyester,polyacrylonitrile and polyamide fibers, glass fibers and aluminumthreads. Fabrics formed of these materials and impregnated with thecompositions of the present invention form outstandingly toughstructures when the composition is thermoset.

5. The bonds possess unusually good resistance to water, salt spray andatmospheric humidity.

The rubbery polymer which is present in the adhesive compositions of thepresent invention may be poly-epichlorohydrin itself, or the polymer maybe a copolymer of epichlorohydrin and a different alkylene monooxidecopolymerizable therewith. The polymer therefore comprisesepicholorohydrin linkages having the theoretical formula:

and, taking the linkages derived from ethylene oxide and propylene oxideas examples, it may also contain linkages having the theoreticalformulae:

(2) CHzCHz-O- and (3) C H2 C HO Alkylene mono-oxides other than ethyleneand propylene oxide may also be employed. Suitable alkylene monooxidesare those which, when copolymerized in minor molar ratio withepichlorohydrin, a yield a water-insoluble polymer having a toughnessand resiliency which re-' sembles crepe rubber.

The epichlorohydrin polymer present in the compositions of the presentinvention is of high molecular weight. The reason why a high molecularweight is necessary has not been ascertained and we do not wish to bebound by any theory. We believe, however, that when the polymer is ofhigh molecular weight it possesses a special compatibility with theepoxy-amine components in that as the epoxy-amine components thermosetto form a uniform adhesive phase, the epichlorohydrin polymerprecipitates into discrete zones with the adhesive phase and that itthereby forms a composition of special morphology which in preferredembodiments possesses an unusually high capacity for the absorption ofstress and strain, and which thus provides superior toughness to thecomposition.

The smallest epichlorohydrin polymer which provides improved toughnessto a practical extent has a molecular weight of roughly 100,000(corresponding to about 1,100 polymer units in the case ofpolyepichlorohydrin and somewhat more in the cases of polymers formed ofepichlorohydrin and alkylene oxy linkages). Even at this comparativelylow molecular weight the polymer is definitely a rubber. It possessessubstantially no surface tack and resembles crepe rubber to the touch.The present invention includes compositions which contain the polymer ofthis molecular weight.

The polymer provides better toughening action as its molecular weightincreases to the point where the polymer is as viscous even in admixturewith a large proportion of solvent that it is unduly difiicult to formit into a homogeneous blend with the adhesive components of the mix.

The maximum useful molecular weight for the polymer has not beendetermined but evidently is in excess of 5,000,000. Because of the goodmixing and toughening properties possessed by the polymer in itsmolecular weight range of 500,0005,000,000, the polymer in this range ispreferred.

At low molecular weight (i.e. at a molecular weight where the polymer isa liquid or a non rubber solid) the polymer does not providecompositions having the principal properties of the compositions of thepresent invention. Thus, when a homogeneous mixture of epoxyamineadhesive components and a low molecular weight epichlorohydrin polymerthermosets, the low molecular weight epichlorohydrin polymer remains inadmixture with the adhesive phase and does not provide the specialmorphology referred to above. It thus acts only as a conventional,soluble plasticizer. This action is detrimental because it causes asignificant decrease in the strength of the adhesive phase at room andat elevated temperatures. A property of primary importance for theadhesive is strength.

The alkylene mono-oxide linkages, when present, act as spacers ordiluents for the epichlorohydrin linkages, and in general the polymers,on an equivalent basis at equal molecular weight, decrease ineffectiveness as the number of such linkages increases. We preferpolyepichlorohydrin itself, therefore, but we include the copolymerswithin the scope of the invention because the epichlorohydrin linkagesare effective in small proportion (e.g. about mol percent). The diluentlinkages thus permit a decrease in the amount of more costlyepichlorohydrin needed without undue sacrifice in thetoughness-imparting properties of the epichlorohydrin linkages. In thepresent specification, therefore, the term epichlorohydrin polymerincludes polyepichlorohydrin itself and copolymers of epichlorohydrinwith major amounts, up to 90 mol percent, of at least one alkylenemono-oxide copolymerizable therewith.

The compositions of the present invention are conveniently prepared bymixing a polyfunctional epoxide, a polyfunctional amine, a rubberyepichlorohydrin polymer and suflicient of a volatile mutual inertsolvent to convert the mixture to a syrup of a convenient viscosity. Themixture is stirred until homogeneous. An elevated temperature ispreferred as this decreases the amount of solvent needed for thedevelopment of a composition of stirrable viscosity.

The mixing is more conveniently performed at elevated temperature, forexample 150 F. The resulting syrup may be used at once at thattemperature, but should be cooled to room temperature and preferablylower if stored. The solvent can be evaporated under vacuum.

If desired, the resulting syrup can be extruded hot as a thin coatingupon release paper and the resulting coated paper can be wound into aroll. Most of the solvent volatilizes as the syrup leaves the extrusionslot. When cool, the adhesive is a strong rubbery gel which can bepeeled intact from the release paper. It can be handled conveniently butpossesses sufiicient tack so that it adheres firmly when pressed againstthe underside of stainless steel, aluminum, nickel, copper and chromiumsheeting.

Textile (cloth) composed of glass, Orlon, Dacron, aluminum, nylon andstainless steel fibers can be impregnated with the hot syrup, andstructures of great hardness and toughness develop when the adhesivecontent thereof is thermoset. The thermoset cloth, before thermosetting,can be employed as adhesive in the same manner as the unsupported film,but the cloth acts as spacer, maintaining the surfaces to be laminatedat a predetermined distance apart.

As little as 10% of the epichlorohydrin polymer, based on the combinedweights of the epoxide and amine components, produces a substantialimprovement in the toughness of the adhesive composition, so thatevidently there is no amount, however small, which will not produce at 4k I least some benefit. The maximum effective amounts has not beenascertained, but amounts at least up to 40% of the weight of the epoxideand amine components have also produced improvements in toughness.However, the optimum amount in any instance can be found by laboratorytrial, by methods illustrated in the examples. 7

The present invention appears to be primarily a physical one, in that wehave no evidence that the rubbery polymer reacts with the othercomponents of the composition either before or after they are thermoset.The invention, therefore, appears to be of general applicability and isnot limited to any particular group of epoxides or amines.

The epichlorohydrin polymers present in the adhesive compositions of thepresent invention possess at most only a negligible proportion ofreactive substituents. Thus, the lowest molecular weight polymer ofpractical use within the scope of this invention (molecular weight100,000) contains only about 0.2 mol percent of reactive substituents.On the other hand, polymers having molecular weights of 500,000 and2,800,000 respectively impart very satisfactory toughening effect butrespectively contain only about 0.044 and 0.006 mol percent of reactivesubstituents. These percentages are far too small to explain thepronounced toughening effect which the polymers impart to epoxy-amineadhesive compositions, and so should be neglected.

The invention is further illustrated by the examples which follow. Theseexamples are best embodiments of the invention and are not to beconstrued as limitations thereon. 1

EXAMPLE 1 a molecular weight of about 2,800,000 as a 10% solution inmethylene chloride, followed by 4 parts of dicyandiamide and 2 parts ofa bisurea [the 1,l'-(4-methyl-mphenylene)-bis(3,3-dimethylurea)] ofNawakowski et al. US. Pat. No. 3,386,955 as cohardeners. The productis ahomogeneous mixture of the components or low condensation products ofthe bisphenol A with the dicyandiamide and the urea, or a mixture of'the components 'with the low condensation product.

A part of the resulting syrup is then extruded as a film upon releasepaper and heated briefly to volatilize the solvent. The resulting film(the test film) peels easily from the paper. The film is strong andself-sustaining, and possesses excellent cold F.) tack properties,remaining firmly in place when pressed against the underside of ahorizontal aluminum sheet.

A control film is prepared in the same manner except that it containsnone of the toughening polymer.

EXAMPLE 2 The procedure of Example 1 is repeated except that the amountof toughening agent is increased to 15%.

EXAMPLE 3 The procedure of Example 1 is repeated except that the amountof toughening agent is increased to 20%.

EXAMPLE 4 The procedure of Example 1 is repeated except that the amountof toughening agent is increased to 25%.

EXAMPLE 5 The bonding properties of the adhesives of Examples 1-4 inthermoset state are determined as follows:

The shear strength (dry) of the adhesive is determined by overlapping/2" of the ends of two 4" x 12" strips of cleaned and acid-washedaircraft grade aluminum 0.064" in thickness and placing in the resultinglap (4" x /2") an appropriately sized piece of the adhesive film to betested. The assembly is subjected to a pressure of 40 lb./ in. and atemperature of 250 F. for one hour (which thermosets the adhesive), andis then cut into strips of suitable width for testing. The shearstrength is determined by straight line pull, and is the pull (poundsper square inch of lap) necessary to cause the bonded area (the joint)to fail.

Shear strength (wet) is determined in the same manner except that afterthe adhesive has thermoset the laminate is boiled in water for threedays and the sample is immersed in water during the straight line pullto ensure that the bonded area is of maximum wetness.

Peel strengths are determined on laminate prepared in the same mannerexcept that the 4" x 12" strips are completely overlapped and sufficientadhesive is used so that the bonded area extends from one end of theassembly to the midpoint thereof. The cemented portion thus has an areaof 4" x 6", and half of the area of the assembly is uncemented. For theclimbing drum test one of the strips of the assembly (at a place whereit is uncemented) is bolted against a vertical steel plate and the otherstrip is peeled upwardly away from the bolted strip against an unfixedsteel roller or drum 3" in diameter which climbs up along the assemblyas the peeling action progresses.

Bell peel strength is determined by peeling both laminae of an assemblysuch as is used for the climbing drum test between two fixed rollersseparated by a distance equal to the thickness of the laminate. Thelaminae are peeled apart against these rollers, each of Which thus actsas a mandrel for one of the laminae.

The tests are performed over a wide temperature range illustratingtemperatures experienced by aircraft in operation.

The adhesives used are those of Examples 1, 2, 3 and 4, and are testedagainst a control which is prepared by the method of Example 1 exceptthat it contains none of the toughener.

Results are as follows:

Parts toughener in compn.

None

(control) F Shear, wet b (after y boil), 73 Peel, climbing drum, 73 F-Peel, bell: c

3 15c; 100 parts by weight of adhesive components. See Examples 1, 2

b Lb. of pull per square inch of lap.

Lb. per inch of width of laminate.

d Sheets virtually fall apart at start of peel.

EXAMPLE 6 EXAMPLE 7 Samples of nylon, polyester (Dacron), polynitrile(Orlon) and glass fiber cloth are impregnated by immersion in the syrupof Example 1. Excess syrup is wiped ofi with a glass rod, and thesolvent is evaporated at F.- F. The resulting samples of laminated clothcan be readily cut with scissors, and the shapes thus made possess verygood cold tack. They are as effective adhesives as the unsupported filmsof Example 1-4.

EXAMPLE 8 Samples of the impregnated cloths of Example 6 are thermocuredby being placed in an oven at 275 F. for 1 hour. The samples was verystrong. The sample containing the fiberglass cloth possessesoutstandingly high strength.

EXAMPLE 9 The procedure of Example 1 is repeated except thatepichlorohydrin-ethylene oxide copolymer is prepared by copolymerizingthe two materials in 7 :3 molar ratio. The adhesive film possessessubstantially the same properties as the test film of Example 1.

EXAMPLE 10 The procedure of Example 1 is repeated except that theepichlorohydrin-ethylene oxide copolymer is prepared by copolymerizingthe two materials in 3:7 molar ratio. The adhesive film possessessubstantially the same properties as the test film of Example 1.

We claim:

1. A thermosetting epoxy-amine adhesive composition composed of ahomogeneous mixture of polyfunctional epoxides having at least two epoxygroup per molecule and polyfunctional amines or low condensationproducts thereof, said composition having an efiective uniformlydistributed dissolved content as latent toughening agent of awater-insoluble non-tacky rubbery polymer having a molecular weight inexcess of 100,000 selected from the group consisting ofpolyepichlorohydrin and copolymers of epichlorohydrin with up to 90 molpercent of at least one alkylene mono-oxide copolymerizable therewith,the weight of said polymer being at least about 10% of the weight of theepoxy-amine component in said composition.

2. A composition according to Claim 1 wherein the toughening agent ispolyepichlorohydrin.

3. A composition according to Claim 1 wherein the toughening agent is acopolymer of epichlorohydrin and ethylene oxide in about 60:40 molarratio.

4. A composition according to Claim 1 wherein the toughening agent is acopolymer of epichlorohydrin and propylene oxide in about 60:40 molarratio.

5. A composition according to Claim 1 wherein the molecular weightof'the toughening agent is in the range of 500,000 to 5,000,000.

6. A composition according to Claim 1 in film form.

7. A textile impregnated with a composition according to Claim 1.

8. Fiberglass cloth impregnated with a composition according to Claim 1.

9. A laminate consisting essentially of aluminum sheets bonded togetherby a composition according to Claim 1 in thermoset state.

References Cited UNITED STATES PATENTS 3,058,921 10/1962 Pannell 260830TW 3,365,516 1/1968 Prescott 260830 R 3,281,491 lO/l966 Smith 260-830 TW3,477,966 11/1969 Doss 260-830 TVV OTHER REFERENCES Chemical Abstracts,vol. 73, 1970, Abstract No. 4552111.

PAUL LIEBERMAN, Primary Examiner U.S. Cl. X.R.

117126 GE, 132 BE, 138.8 F, 138.8N; 1 61-484, 186; 26'083OTW Patent No.5 830 817 Dated nglst 2Q 95 Inventor(s) JAMES FRANCIS vAsIL ere 5osEiPHs. BURSEL It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

Column 2 line 4-0. After epichlorohydrin," delete a Column 2 line 52.Change "with" to within Column 4 line 1. Change "amounts" to amountSigned and sealed this 3rd day of December 1974.

(SEAL) Attest;

McCOY M. GIBSON JR. 0. MARSI:IALL DANN Attesting Officer Commissioner ofPatents 1 USCDMM'DC GOSTG POQ 0.5. GOVERNMENT PRINTING OFFICE 1!!O-lGi-Sll FORM PO-IDSO (10-69)

